1. Field of Invention
The invention relates to an automatic steering device and method employed in a vehicle automatically driven for parking, for example, without requiring a vehicle operator to perform a steering operation.
2. Description of Related Art
An automatic steering device disclosed in Japanese Patent Application Laid-Open publication No. 4-205505 allows the operator of the vehicle to perform the steering operation easily during the automatic steering operation by generating automatic steering torque and an assist torque of the power steering simultaneously. The automatic steering device reduces the steering torque in excess of the automatic steering torque using the assist torque of the power steering. When the vehicle operator intends to perform the steering operation in the automatic steering mode, the steering torque is reduced so as to assist the vehicle operator to perform the steering operation easily.
When the vehicle operator performs steering operation in the automatic steering mode, the steering mode is switched to the manual steering mode by cancellation of the automatic steering mode. According to the technology disclosed in the aforementioned publication, the automatic mode can be smoothly switched to the manual steering mode. As the automatic steering device that has been produced before introduction of the aforementioned technology generates no assist torque of power steering in the automatic steering mode, the automatic steering torque is lost at substantially a high rate. The assist torque, in turn, is generated at a high rate in the direction opposite to that of the automatic steering torque, resulting in a sharp change in the steering reaction force. On the contrary, the technology disclosed in the aforementioned publication prevents sharp change in the steering reaction by reducing the steering torque in excess of the automatic steering torque. As a result, the steering mode can be smoothly switched between the manual steering mode and automatic steering modes.
The technology for switching the steering mode from the automatic steering mode to the manual steering mode in response to the steering operation of the vehicle operator is disclosed in JP-A-11-198839. In this technology, it is determined whether the steering operation has been performed by the vehicle operator based on detection values of the steering torque detected by the steering torque sensor.
With the automatic steering device that generates the automatic steering torque and the assist torque of power steering simultaneously, the vehicle operator is allowed to perform the steering operation even in the automatic steering mode at a reduced steering torque. The steering torque that has been kept small for assisting the vehicle operator may, in turn, make it difficult to determine whether the steering operation has been performed based on the steering torque detection values. The threshold value of the steering torque detection values have to be decreased so as to determine the steering operation of the vehicle operator. In this case, however, it is difficult to distinguish the steering torque generated by the steering operation of the vehicle operator from an inertia torque of the steering wheel itself as noise. As a result, the steering operation cannot be accurately detected.